1. Field of the Invention
This invention relates generally to apparatus, such as a video tape recorder (VTR) of the so-called "helical scan type", in which video or other information signals are recorded in successive parallel tracks which are skewed or extend obliquely on a magnetic tape and, more particularly, is directed to an improved tracking control system by which a magnetic head or other transducer in such apparatus is made to accurately scan the track or tracks in which the video or other information signals are recorded.
2. Description of the Prior Art
In a helical-scan VTR, the magnetic tape extends helically about at least a portion of the periphery of a guide drum and is adapted to be moved or advanced in the longitudinal direction of the tape while at least a portion of the guide drum is rotated, and the transducer or magnetic head is mounted on a rotated portion of the guide drum so as to rotate with the latter and thereby repeatedly scan across the tape in a path at an angle to the longitudinal direction of the tape. During the recording operation of the VTR, the angle between the scanning path, and hence each record track, and the longitudinal direction of the tape is dependent on the rotational speed of the rotary head and also the speed at which the magnetic tape is longitudinally advanced. Accordingly, if the speed and direction of advancement of the magnetic tape are not the same during the reproducing operation as during the recording operation, then the scanning path of the magnetic head during reproducing will not precisely follow or coincide with a record track on the tape during each movement of the head across the tape and, accordingly, the recorded video or other information signals may not be correctly or accurately reproduced.
Various tracking control or servo systems have been proposed for maintaining correct tracking or scanning of the record tracks by the rotary head. In the most desirable of these known arrangements, means are provided for deflecting the head in a direction normal to the plane of its rotation, that is, in a direction which is transverse in respect to the direction along each of the record tracks, and the amount of such deflection is electrically controlled during the movement of the head along each track so as to achieve accurate scanning of the latter.
Generally, the transducer deflecting device in the above systems is constituted by an electro-mechanical element, such as a bi-morph leaf, which is contructed by laminating two piezo-ceramic leaves with at least one electrode plate, and which is deflected in accordance with a drive voltage supplied thereto. For example, in U.S. Pat. application Ser. Nos. 6/073,246, 6/152,117, 6/232,014, and 6/243,352 (now U.S. Pat. No. 4,370,685), filed Sept. 6, 1979, May 21, 1980, Feb. 6, 1981, and Mar. 3, 1981, respectively, all having a common assignee herewith, there is disclosed an automatic tracking control system in which the drive signal applied to the transducer deflecting device comprises a jump signal or track selection control signal for determining the starting position of the transducer during non-normal or special reproducing modes and, thereby, the next desired track to be followed or scanned thereby, a dithering signal or oscillation, a track following error signal, and a slant angle correction signal. In this manner, the tracking error between the recorded track on the magnetic tape and the scanning path of the reproducing head is corrected to effect coincidence therebetween. A reproduced picture of high quality without any guard band noise can thus be obtained even when the tape speed is varied during various special reproducing modes, such as during slow-, still-, fast- or reverse motion-playback modes.
Generally, the record tracks are formed along a curved path on the tape rather than an ideal straight line path. Thus, each record track, which includes one field of video information, has a small skew distortion which results from distortion of the tape and the like. In particular, the beginning end and the concluding end of each track deviates from and is inclined with respect to the ideal straight line path of the respective record track. Because of the nature of the inclination at the beginning and concluding ends of each track, the magnetic head, after scanning the concluding end of a track, is offset when it begins scanning the beginning end of the next adjacent track. This is compensated for in the above-mentioned U.S. patent applications by means of the track following error signal for bringing the magnetic head into alignment with the track then being scanned thereby. However, with such method of correction, the offset deviation is corrected only after the magnetic head has begun scanning the track. Thus, an accurate scanning of the track may only occur after the passage of a number of horizontal line intervals due to, for example, the delay in error detection of the tracking control system and the delay in response to the bi-morph leaf to the correction signal supplied thereto. In addition, with such offset deviation, a step pulse signal is supplied to the bi-morph leaf for effecting the correction thereof. However, such step pulse signal causes the bi-morph leaf, and therefore, the magnetic head secured thereto, to overshoot the record track. Accordingly, a transient vibration of the bi-morph leaf is produced which is slowly attenuated during the scanning operation and which results in a further deviation of the scanning trace of the magnetic head from the beginning of the record track then being scanned thereby. Thus, the beginning end of the track is not accurately scanned, which may result in a portion, for example, the top portion, of the reproduced picture being distorted.
In the case of special reproducing modes, that is, where the tape speed during reproduction is different from the tape speed during recording, the pitch of the track to be scanned is different from the pitch of the scanning trace of the magnetic head. In such case, it is necessary to effect tracing track selecting in order to perform overlap-tracing or interval-tracing. Overlap-tracing is required in a slow-motion playback mode where the tape speed is slower than the recording tape speed, and in which the head repeatedly traces or scans a recorded track and then the head scans the next track on the tape. Interval-tracing or skip-tracing is required in a fast-motion playback mode employing a tape speed faster than the recorded speed, and in which one or several tracks are skipped and the head only scans the tracks at intervals. The alteration of the head from the end of the track being traced to the beginning of the next desired one in the overlap-tracing or interval-tracing mode is called "track jump" or "flyback". The time period within which the reproducing head performs the jump or fly-back operation, is extremely short. Accordingly, a steep step or inclined voltage is applied to the bi-morph leaf as the jump or fly-back voltage therefor and, as a result, the transient vibration of the head becomes greater. This, of course, results in increased error or mistracking at the beginning end of the track.
It has been proposed, for example, in U.S. Pat. No. 4,148,082, having a common assignee herewith, to use a pilot signal which is superimposed on the horizontal synchronizing signals and which has a frequency that cyclically changes for successive horizontal blanking intervals. By means of the pilot signal, the tracking error between the scanning trace of the magnetic head and the track being scanned thereby can be determined. In this apparatus, such tracking error is detected at the beginning end of each track and stored, and is used to correct tracking errors at the beginning end of the next scanned track. The stored signal is supplied to the bi-morph leaf during the time when the head is not scanning a track, whereby the transducer is pre-displaced by the stored tracking error signal prior to beginning its scan of the next adjacent track. Since the deviation of a track from its ideal straight line path at the beginning and concluding ends thereof is generally the same for each track, the above pre-displacement of the magnetic head has the effect, to some extent, of reducing the tracking error at the beginning end of each track. However, even with this system, errors are still present in the tracking operation at the beginning end of each track. For example, because of the large noise component generally accompanying the tracking error signal, the stored tracking error signal does not correspond to the actual error at the beginning end of each track. Accordingly, after the initial pre-displacement of the magnetic head has been performed, a hunting operation must be performed in order for the magnetic head to accurately scan the track. In this regard, this system is similar to the systems of the aforementioned co-pending U.S. patent applications in that accurate scanning at the beginning end of the track is not achieved. Further, if there is a break in the similarity of the deviations at the beginning and concluding ends of the tracks, that is, if the beginning end of a track has a deviation from its ideal straight line path that is much greater than those of the remaining tracks, the tracking control system is greatly affected thereby so that accurate tracking may not be resumed until the magnetic head has scanned numerous tracks. Also, slight variations in the time when the tracking error signal is sampled at the beginning end of each track will have a significant effect on the scanning operation of the next track to be scanned.